Is this in a room? Or in here on forums?
If it's in a chat room and they were a guest, they can re-enter the room and contact you again - the only way around that is to limit who can PM you.
If a user in a chat room is placed on ignore, and they have an account in any #1 Chat Avenue chat room, they cannot contact you 100% of the time.
The only exception to the above is the Video Chat room. It runs on a different type of software but is also similar with its ignore features.
If it's on forums, the ignore feature works completely well unless they make a new forums account and you identify them as the same person you are ignoring.